The invention relates to a process and device for continuously controlling the degree of locking of externally controlled locking differentials in a driven axle of a multi-axle vehicle. Externally controlled locking differentials, in this context, are in contrast to standard torque or speed sensing locking differentials.
Locking differentials with external control means usually comprise plate couplings by means of which the open differential may be locked. By using suitable adjusting members it is possible to continuously vary by hydraulic means the locking torque transmitted by the plate coupling. This is important in order to make it possible, largely by way of electronic control, to adapt the locking torque in respect of traction and driving stability to the different slip and friction conditions between the tires and the road surface. With the help of speed sensors at the driven and non-driven axle it is possible to identify the slip conditions of the driving wheels and to prevent wheel spinning by controlling the locking differential.
There are prior art regulating processes which, as a function of the driving situation determined by the wheel speed sensors, set a certain locking effect, and for different driving situations there exist a multitude of different reactions demonstrated by the locking coupling. In view of the different relationships between interfering factors, this leads to complex and therefore intricate and expensive control strategies.
With prior art differentials of the above type, a high degree of slip at the driven axle leads to the locking coupling being completely closed so that with a 100% locking effect both wheels of the driven axle rotate at exactly the same speed. In such a case, evaluation of the speed no longer allows it to be determined whether there is still a need for actuating the locking coupling. Therefore, changes in gripping conditions, wheel load, steering angle or driving torque (interfering factors) can no longer be taken into account for adapting and setting the correct locking torque. Temporarily eliminating the locking effect in order to use the behavior of the wheel speeds for calculating the actual or corrected locking effect for the next time interval would be disadvantageous for driving stability because of a locking torque which increases and decreases at intervals. Actuating a controlled locking differential as described above is particularly disadvantageous for front wheel drive vehicles because of negative steering effects.